General request regarding render engine
OptiX

Quick answer is:


To take advantage of RTX hardware raytracing, select OptiX
otherwise GPU + CPU is mostly faster than pure GPU if you select the CUDA option, unless you would have a very weak CPU


Official blender announcement one year ago

https://code.blender.org/2019/07/accelerating-cycles-using-nvidia-rtx/

Some benchmarks by random users

https://www.youtube.com/watch?v=VXibUlLXwKk

https://www.youtube.com/watch?v=LRC34eO2AiE

https://www.reddit.com/r/blender/comments/cwx14w/some_rtx_optix_benchmarks/

Render settings

Regarding the render settings, that is very subjective and highly depends on the lights setup, types of materials you use and other factors. I can't answer that in a general sense. The defaults are good for start. Diffuse-only scenes with simple direct lights will look fine with much less samples per pixel than complex scenes with glossy objects and many lights or indirect lighting only. The new adaptive sampling in 2.83 will help towards more universal settings, but it will still remain a matter of experience and subjective judgement.